Metallocene catalyst compounds are well known olefin polymerization catalysts. While a variety of different techniques may be used to synthesize suitable metallocene catalyst compounds, one technique involves the use of alkyl cyclopentadiene compounds, such as n-propylcyclopentadiene and n-butylcyclopentadiene. Unbridged metallocene catalyst compounds containing at least one n-alkyl cyclopentadienyl ligand (propyl or longer) can show increased productivity over metallocene catalyst compounds that do not contain this group. This increase in productivity has been referred to as the “propyl effect.”
One technique for synthesizing n-alkyl cyclopentadienes involves a fulvene intermediate. The fulvene intermediate may be reduced, for example, with LiAlH4 to produced substituted cyclopentadienide which can then be used directly in the synthesis of the metallocene catalyst compound. This technique may be problematic, however, in that it may be difficult to separate the desired cyclopentadienyl compound from the aluminum containing byproducts. An aqueous work-up is often required to achieve the separation which requires isolation of the free cyclopentadiene derivative.
Another technique for synthesizing n-alkyl cyclopentadienes is the reaction of a cyclopentadiene nucleophile with an electrophile, such as an alkyl halide. However, the yield and subsequent purity of the product are highly dependent upon reaction conditions. For example, sodium cyclopentadienide in tetrahydrofuran (“THF”) can react readily with alkyl bromides at room temperature, but the reaction typically results in undesirable levels of impurities and yields of the desired product can be low.
N-alkyl cyclopentadienes can be produced more cleanly by reacting alkyl trifluoromethylsulfonate with lithium cyclopentadienide in THF or reacting 1-iodobutane with sodium cyclopentadienide in liquid ammonia. In addition, substituted cyclopentadienes may be produced by reacting sodium cyclopentadienide with alkyl halides in liquid ammonia. However, despite higher yields, these additional techniques require the use of liquid ammonia at low temperature and the use of alkyl trifluoromethanesulfonates which are expensive, air and moisture sensitive, toxic and may not be readily available.
Yet another technique for synthesizing n-alkyl cyclopentadienes involves use a cyclopentadienyl Grignard reagent. For example, cyclopentadienylmagnesium bromide can be reacted with iodomethane to form methylcyclopentadiene. In some instances, an aqueous acid may be used to quench the reaction. However, drawbacks from this procedure include, higher levels of impurities from the aqueous work-up.
Accordingly, there is a need for improved methods of synthesizing alkyl cyclopentadiene compounds.